Parathyroid hormone-related protein (PTHrP) is a normal product of mammary epithelial cells, and previous studies have demonstrated that targeted under- or over expression of PTHrP leads to severe defects in mammary development during embryonic and post-natal mammary development, respectively. We have recently shown that transient over expression of PTHrP before birth causes defects in ductal branching during puberty and that over expression of PTHrP during puberty causes defects in ductal elongation. The impairment in ductal elongation caused by PTHrP is associated with and increase in the basal rate of epithelial cell apoptosis in terminal end buds and a failure to increase end bud cell proliferation in response to estrogen and progesterone. These findings suggest that PTHrP has a dual role in the mammary gland. Before birth, PTHrP is involved in determining the subsequent pattern of epithelial branching during puberty. After birth, PTHrP is involved in regulating hormone-induced ductal elongation at the terminal end bud. The objective of this proposal is to understand the mechanisms by which PTHrP-over expression results in defects in mammary epithelial cell growth during puberty. We have designed three specific aims that involve using a cell culture system consisting of primary cultures of mouse mammary epithelial and stromal cells from wild type and PTHrP overexpressing mice to meet this objective. First, we will determine if the defect in ductal branching in the PTHrP overexpressing mice is due to the effects of PTHrP on mammary epithelial cells themselves, or if this defect is the result of PTHrP signaling through the mammary stroma. Second, we will examine the effects of PTHrP on the hormone-induced growth of mammary epithelial cells in culture. Finally, we will examine the effects of PTHrP on the expression and/or activation of several growth factors and/or growth factor receptors in the mammary gland. It is our expectation that the results from these experiments will lead to a greater understanding of the role of PTHrP in the post-natal development of the mammary gland, and also in the development of other epithelial organs. In addition, this work will have a broad impact on science education at the undergraduate level, as the experiments described in this proposal have been designed specifically for undergraduate researchers.